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Spectral heterogeneity of oxygen-deficient centers in Ge-doped silica
Radiation Measurements 38 (2004) 645 – 648 www.elsevier.com/locate/radmeas
Spectral heterogeneity of oxygen-de&cient centers in Ge-doped silica Simonpietro Agnelloa;∗ , Roberto Boscainoa , Marco Cannasa , Andrea Cannizzoa , Franco M. Gelardia , Stefania Grandib , Maurizio Leonea a Department
of Physical and Astronomical Sciences, Istituto Nazionale per la Fisica della Materia, Via Archira 36, Palermo I-90123, Italy b Department of Physical Chemistry, Istituto Nazionale per la Fisica della Materia, Via Taramelli 16, Pavia I-27100, Italy Received 4 November 2003; received in revised form 4 November 2003; accepted 7 December 2003
Abstract We report an experimental investigation of the emission spectra of a 1000 mol ppm sol–gel Ge-doped silica by &ne tuning the excitation energy in the ultraviolet (UV) range, around 5 eV, and in the vacuum-UV range, around 7:3 eV, at room temperature and at 10 K. The sample is characterized by a blue (centered at ∼ 3:2 eV) and an UV (centered at ∼ 4:3 eV) bands. We have found that the ratio between the area of the blue and the UV bands depends on the temperature and on the excitation energy in both the vacuum-UV and the UV range. At both temperatures the spectral features of the blue and the UV bands are weakly a;ected when the excitation is varied in the vacuum-UV. At variance, under UV excitation the peaks of the bands are shifted and also their widths are changed. These results are interpreted in terms of distinct excitation channels of the luminescence that are in
1. Introduction Ge-doped amorphous SiO2 is a material of large interest due to its use in optical &bers. Much interest arises from its enhanced photosensitivity that enables to realize on &ber optical devices as Bragg gratings, <ers and mirrors (Askins, 2000). This photosensitivity has been connected to Ge-related oxygen-de&cient centers characterized by an optical activity (B-type) (Neustruev, 1994), consisting in an absorption band centered at 5.1–5:4 eV, named B2 , and two related emission bands centered at 4.2–4:3 eV, ultraviolet (UV) band, and at 3.0 –3:2 eV, blue band, named E and , respectively (Skuja, 1992, 1998). The E band is attributed to the inverse of the absorption transition and the band is related to a triplet to singlet transition fed by a phonon-assisted intersystem-crossing (ISC) process. It ∗ Corresponding author. Tel.: +39-091-6234255; fax: +39-091-6162461. E-mail address: agnello@&sica.unipa.it (S. Agnello).
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2003.12.014
has been evidenced that the spectral characteristics of these emission bands depend on the excitation energy inside the absorption band (Martini et al., 1998; Leone et al., 1999; Trukhin et al., 1999; Anedda et al., 2002). This feature has been attributed by some authors to the coexistence of structurally di;erent defect types and by other to the structural inhomogeneity arising from the amorphous matrix. The E and bands can be excited also in the vacuum-UV (v-UV) energy range as reported by Corazza et al. (1995), Crivelli et al. (1996) and Anedda et al. (2003). However, the study of their dependence on the excitation energy has been poorly considered in the v-UV range where the spectral inhomogeneity can be further investigated in relation with the higher excited electronic states, according to the electronic energy level scheme proposed by Skuja (1992), that attributes the optical features of the B-type activity to a single defect a;ected by the environment inhomogeneity. Besides, new hints can be found to clarify the features of the di;erent bands underneath the E and bands proposed by Corazza et al. (1995), Martini et al. (1998) and Anedda et al. (2002).
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
In this work a study of the photoluminescence (PL) bands related to the optical B-type activity in a sol–gel derived Ge-doped silica is reported. Synchrotron radiation light is used for &ne tuning excitation in UV and in v-UV range aiming to compare the excitation energy dependence of the emission bands. Moreover, to investigate the role of the phonon-assisted ISC relaxation mechanisms, measurements were performed at two distinct temperatures: T = 10 K and 300 K. 2. Experimental details Measurements were done in a sample of Ge-doped silica with a nominal Ge content of 1000 mol ppm obtained by a sol–gel route and featuring the characteristic absorption and emission bands of the B-type activity (Grandi et al., 2003). Sample is disk shaped with 4:0 mm diameter and 0:9 mm thickness, with surfaces optically polished. The B2 absorption band has amplitude ∼ 14 cm−1 corresponding to a concentration of B-type centers of ∼ 1017 cm−3 , estimated from the absorption cross section at 5:1 eV (Fujimaki et al., 1999; Agnello et al., 2000). Measurements were carried out at 10 and 300 K in a sample chamber equipped with a cold &nger &tted to a continuous
45
PL amplitude (arb. units)
646
40
T = 300 K
35 30 25 20
90 80 70 60 50 40 30 20 10 0
T = 10 K
3.0
3.5
4.0
4.5
Energy (eV)
15 10 5 0
3.0
3.5
4.0
4.5
Energy (eV) Fig. 1. Photoluminescence spectra at T =300 K with excitation energies: 6:9 eV (solid line), 7:2 eV (dash–dot–dot), 7:5 eV (dashed) and 7:7 eV (dash–dot). In the inset, the spectra at the same excitation energies are reported for T = 10 K.
is not done because the spectra are distorted by the high absorption of the sample. In Fig. 1, emission spectra at T = 300 K are reported for excitation energies in the v-UV range. It is observed that the band maximum position is subjected to small shift together with small variation of the FWHM on increasing the excitation energy. Analogous results are found for the E band. Besides, both bands feature a reduction of amplitude on increasing the excitation energy. Measurement at 10 K are reported in the inset of Fig. 1. Small variation of the maximum position and the FWHM are evidenced also at this temperature. The PL amplitude has non-monotonic dependence on the excitation energy as is well evident for the E band. Fig. 2 shows the position of the band maximum as a function of the excitation energy in the v-UV range from 6.9 up to 7:7 eV. At T = 300 K the position varies from 3.15 up to 3:18 eV and the FWHM (data not shown) changes from 0.44 to 0:46 eV. At T =10 K, we &nd that the maximum position changes from 3.13 to 3:15 eV and the FWHM changes from 0.36 to 0:39 eV. In the inset of Fig. 2, the dependence of the maximum position of the band is reported as a function of the excitation energy in the UV range from 4.6 up to 5:5 eV. Both at T = 300 and 10 K the maximum position changes almost monotonically from 3.07 up to 3:20 eV. At T = 300 K the FWHM increases from 0.38 to 0:51 eV increasing the excitation energy. At T = 10 K, where the band is clearly detectable for excitation energies from 5.0 up to 5:5 eV, FWHM increases from 0.38 to 0:46 eV. The dependence of the E band on the excitation energy is reported in Fig. 3. At T = 300 K, the position of the maximum changes in the range from 4.31 to 4:33 eV and the
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
η (area αE/ area β)
0.65 3.15
3.10
4.6
4.8
5.0
3.20
5.2
5.4
Eexc (eV)
η (area αE/ area β)
3.25
0.70
3.20
Peak Energy (eV)
Peak energy (eV)
3.30
647
0.60 0.55 0.50 0.45 0.40
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
7.0
7.2
7.4
7.6
Eexc(eV)
0.35 0.30 0.25
3.15
0.20 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
Eexc(eV) Fig. 2. Energy position of the emission maximum of the (blue) band reported as a function of the excitation energy in the vacuum-UV range at T = 300 K (triangles) and at T = 10 K (squares). In the inset, the position of the emission maximum is reported as a function of the excitation energy in the UV range at the same temperatures.
Peak energy (eV)
4.45
4.40
Peak Energy (eV)
4.40 4.35 4.30 4.25 4.20
4.6
4.8
5.0
5.2
5.4
Eexc (eV)
4.35
Eexc(eV) Fig. 4. Ratio, , of the area of the E and emission bands as a function of the excitation energy in the vacuum-UV range at T = 300 K. In the inset, is reported for T = 10 K.
position of the maximum of the E band shows much larger dependence on the excitation energy in the UV range featuring a non-monotonic variation both at T = 300 and 10 K. At variance, the FWHM increases from 0.46 to 0:61 eV at T = 300 K and from 0.38 to 0:56 eV at T = 10 K on increasing the excitation energy. To complete the characterization of the emission of our sample, we report in Fig. 4 the area ratio, , of the E band and the band. As shown, &rst increases and then decreases on increasing the excitation energy in the v-UV range at T = 300 K. At variance, at T = 10 K, increases and then shows a plateau. Under excitation energy in the UV range we found that, on increasing the excitation energy, decreases from 0.75 to 0.3 at T = 300 K, and from 300 to 18 at T = 10 K. 4. Discussion
4.30 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
Eexc(eV) Fig. 3. Energy position of the emission maximum of the E (UV) band reported as a function of the excitation energy in the vacuum-UV range at T = 300 K (triangles) and at T = 10 K (squares). In the inset, the position of the emission maximum is reported as a function of the excitation energy in the UV range at the same temperatures.
FWHM from 0.49 to 0:53 eV. At T = 10 K the energy of the maximum changes from 4.32 to 4:35 eV and the FWHM from 0.44 to 0:50 eV. As reported in the inset of Fig. 3, the
The reported data evidence that under v-UV excitation both the E and bands are detectable at 300 as well as at 10 K. Fine tuning in the range 6.9 –7:7 eV shows that the area ratio of the E and bands changes with the excitation energy. This result suggests that the ISC process that is responsible for the coupling of the excited states from which the E and emissions arise (Skuja, 1992), has a dependence on the excitation energy in the v-UV range as already observed for the UV range (Leone et al., 1999). In this respect, the ISC process re
648
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
composite e;ect on the internal conversion. In fact, at 300 K, &rst increases up to 0.44 and then decreases down to 0.20 whereas, at T = 10 K, always increases up to 1.61 on increasing the excitation energy. It is worth noticing that the ratio under v-UV excitation is typically lower than under UV excitation, especially at low temperature, where the variation of a factor 15 of has been explained with the quenching of the ISC process (Leone et al., 1999; Cannizzo et al., 2003). This result suggests that the higher excited states give a more eLcient intersystem crossing process. The detailed study of the spectral features of the E and bands as a function of the excitation energy in the v-UV range from 6.9 up to 7:7 eV has evidenced that small changes in the position of the maximum and the FWHM of emission bands occur as compared to the e;ects observed under UV excitation, both at low and high temperature. The excitation energy dependence should mainly arise from the inhomogeneity, since under UV excitation it is observed also at low temperature when the ISC is poorly e;ective (Leone et al., 1999; Cannizzo et al., 2003). As a consequence, it is guessed that luminescence excited from the higher energy levels in the v-UV range merges the spectral inhomogeneity of the system. In this respect, it is noted that the maximum position and the FWHM of the E and emission bands when excited in the v-UV range are in good agreement with the average of the values found with UV excitation. Our data can be interpreted assuming one type of optically active center a;ected by glass matrix inhomogeneity. In particular, for excitation energy in the UV range around 5 eV the main process a;ecting the luminescence is an intra center conversion sensible to inhomogeneity of environment. At variance, for excitation in the v-UV range above 6:9 eV a competitive non-intra center excitation occurs, maybe through the conduction band, that is less sensible to inhomogeneity, as suggested by Trukhin et al. (1999). 5. Conclusions We have studied the blue and UV luminescence in Ge-doped silica, arising from B-type activity, by &ne tuning the excitation energy in the vacuum-UV range at 10 and 300 K. We have found that the area ratio of the emission bands depends on the excitation energy whereas the bands spectral features are not a;ected. This result is di;erent from the &ndings for UV excitation energy and it suggests that, under v-UV excitation, inhomogeneity is somewhat merged by a process involving highest excited energy levels. Acknowledgements The authors acknowledge useful discussions with L. Skuja and A. Trukhin. They thank M. Kirm of the G. Zimmerer group and A. Paleari for the measurements time at the
HASYLAB of DESY (Hamburg). This work was partially supported by a National Project (PRIN2002) of the Ministero dell’Istruzione, dell’UniversitNa e della Ricerca, Rome (Italy). One of the authors (S.A.) was partially supported by EC Project ICAI-CT-2000-7007 (CAMART). References Agnello, S., Boscaino, R., Cannas, M., Gelardi, F.M., Leone, M., 2000. -ray induced bleaching in silica: conversion from optical to paramagnetic defects. Phys. Rev. B 61, 1946–1951. Anedda, A., Carbonaro, C.M., Clemente, F., Corpino, R., 2002. Ultraviolet excitation &ne tuning of luminescence bands of oxygen-de&cient centers in silica. J. Appl. Phys. 92, 3034–3038. Anedda, A., Carbonaro, C.M., Clemente, F., Corpino, R., Serpi, A., 2003. Excitation pattern of the blue emission in Ge-doped silica. J. Non-Cryst. Solids 315, 161–165. Askins, C.G., 2000. Periodic uv-induced index modulation in doped-silica optical &bers: formation and properties of the &ber bragg grating. In: Pacchioni, G., Skuja, L., Griscom, D.L. (Eds.), Defects in SiO2 and Related Dielectrics: Science and Technology. Kluwer Academic Publishers, Dordrecht, pp. 391–426. Cannizzo, A., Agnello, S., Boscaino, R., Cannas, M., Gelardi, F.M., Leone, M., Grandi, S., 2003. Role of vitreous matrix on the optical activity of Ge-doped silica. J. Phys. Chem. Sol. 64, 2437–2443. Corazza, A., Crivelli, B., Martini, M., Spinolo, G., 1995. The double nature of the 3:1 eV emission in silica and in Ge-doped silica. J. Phys.: Condens. Matter 7, 6739–6745. Crivelli, B., Martini, M., Meinardi, F., Paleari, A., Spinolo, G., 1996. Excitation channels of the 4:3 eV photoluminescence in Ge–SiO2 . Sol. State Comm. 100, 651–656. Fujimaki, M., Kasahara, T., Shimoto, S., Miyazaki, N., Tokuhiro, S., Seol, K.S., Okhi, Y., 1999. Structural changes induced by KrF excimer laser photons in H2 -loaded Ge-doped SiO2 glass. Phys. Rev. B 60, 4682–4687. Grandi, S., Mustarelli, P., Agnello, S., Cannas, M., Cannizzo, A., 2003. Sol–gel GeO2 -doped SiO2 glasses for optical applications. J. Sol–Gel Sci. Tech. 26, 915–918. Leone, M., Agnello, S., Boscaino, R., Cannas, M., Gelardi, F.M., 1999. Conformational disorder in vitreous systems probed by photoluminescence activity in SiO2 . Phys. Rev. B 60, 11475–11481. Martini, M., Meinardi, F., Paleari, A., Spinolo, G., Vedda, A., 1998. SiO2 :Ge photoluminescence: detailed mapping of the excitation-emission UV pattern. Phys. Rev. B 57, 3718–3721. Neustruev, V.B., 1994. Colour centres in germanosilicate glass and optical &bres. J. Phys.: Condens. Matter 6, 6901–6936. Skuja, L., 1992. Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2 : a luminescence study. J. Non-Cryst. Solids 149, 77–95. Skuja, L., 1998. Optically active oxygen-de&ciency-related centers in amorphous silicon dioxide. J. Non-Cryst. Solids 239, 16–48. Trukhin, A., Poumellec, B., Garapon, J., 1999. Luminescence decay kinetics of Ge related center in silica. Rad. E;. Def. Sol. 149, 89–95. Zimmerer, G., 1991. Status report on luminescence investigations with synchrotron radiation at HASYLAB. Nucl. Instr. Meth. Phys. Res. A 308, 178–186.
Spectral heterogeneity of oxygen-de&cient centers in Ge-doped silica Simonpietro Agnelloa;∗ , Roberto Boscainoa , Marco Cannasa , Andrea Cannizzoa , Franco M. Gelardia , Stefania Grandib , Maurizio Leonea a Department
of Physical and Astronomical Sciences, Istituto Nazionale per la Fisica della Materia, Via Archira 36, Palermo I-90123, Italy b Department of Physical Chemistry, Istituto Nazionale per la Fisica della Materia, Via Taramelli 16, Pavia I-27100, Italy Received 4 November 2003; received in revised form 4 November 2003; accepted 7 December 2003
Abstract We report an experimental investigation of the emission spectra of a 1000 mol ppm sol–gel Ge-doped silica by &ne tuning the excitation energy in the ultraviolet (UV) range, around 5 eV, and in the vacuum-UV range, around 7:3 eV, at room temperature and at 10 K. The sample is characterized by a blue (centered at ∼ 3:2 eV) and an UV (centered at ∼ 4:3 eV) bands. We have found that the ratio between the area of the blue and the UV bands depends on the temperature and on the excitation energy in both the vacuum-UV and the UV range. At both temperatures the spectral features of the blue and the UV bands are weakly a;ected when the excitation is varied in the vacuum-UV. At variance, under UV excitation the peaks of the bands are shifted and also their widths are changed. These results are interpreted in terms of distinct excitation channels of the luminescence that are in
1. Introduction Ge-doped amorphous SiO2 is a material of large interest due to its use in optical &bers. Much interest arises from its enhanced photosensitivity that enables to realize on &ber optical devices as Bragg gratings, <ers and mirrors (Askins, 2000). This photosensitivity has been connected to Ge-related oxygen-de&cient centers characterized by an optical activity (B-type) (Neustruev, 1994), consisting in an absorption band centered at 5.1–5:4 eV, named B2 , and two related emission bands centered at 4.2–4:3 eV, ultraviolet (UV) band, and at 3.0 –3:2 eV, blue band, named E and , respectively (Skuja, 1992, 1998). The E band is attributed to the inverse of the absorption transition and the band is related to a triplet to singlet transition fed by a phonon-assisted intersystem-crossing (ISC) process. It ∗ Corresponding author. Tel.: +39-091-6234255; fax: +39-091-6162461. E-mail address: agnello@&sica.unipa.it (S. Agnello).
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2003.12.014
has been evidenced that the spectral characteristics of these emission bands depend on the excitation energy inside the absorption band (Martini et al., 1998; Leone et al., 1999; Trukhin et al., 1999; Anedda et al., 2002). This feature has been attributed by some authors to the coexistence of structurally di;erent defect types and by other to the structural inhomogeneity arising from the amorphous matrix. The E and bands can be excited also in the vacuum-UV (v-UV) energy range as reported by Corazza et al. (1995), Crivelli et al. (1996) and Anedda et al. (2003). However, the study of their dependence on the excitation energy has been poorly considered in the v-UV range where the spectral inhomogeneity can be further investigated in relation with the higher excited electronic states, according to the electronic energy level scheme proposed by Skuja (1992), that attributes the optical features of the B-type activity to a single defect a;ected by the environment inhomogeneity. Besides, new hints can be found to clarify the features of the di;erent bands underneath the E and bands proposed by Corazza et al. (1995), Martini et al. (1998) and Anedda et al. (2002).
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
In this work a study of the photoluminescence (PL) bands related to the optical B-type activity in a sol–gel derived Ge-doped silica is reported. Synchrotron radiation light is used for &ne tuning excitation in UV and in v-UV range aiming to compare the excitation energy dependence of the emission bands. Moreover, to investigate the role of the phonon-assisted ISC relaxation mechanisms, measurements were performed at two distinct temperatures: T = 10 K and 300 K. 2. Experimental details Measurements were done in a sample of Ge-doped silica with a nominal Ge content of 1000 mol ppm obtained by a sol–gel route and featuring the characteristic absorption and emission bands of the B-type activity (Grandi et al., 2003). Sample is disk shaped with 4:0 mm diameter and 0:9 mm thickness, with surfaces optically polished. The B2 absorption band has amplitude ∼ 14 cm−1 corresponding to a concentration of B-type centers of ∼ 1017 cm−3 , estimated from the absorption cross section at 5:1 eV (Fujimaki et al., 1999; Agnello et al., 2000). Measurements were carried out at 10 and 300 K in a sample chamber equipped with a cold &nger &tted to a continuous
45
PL amplitude (arb. units)
646
40
T = 300 K
35 30 25 20
90 80 70 60 50 40 30 20 10 0
T = 10 K
3.0
3.5
4.0
4.5
Energy (eV)
15 10 5 0
3.0
3.5
4.0
4.5
Energy (eV) Fig. 1. Photoluminescence spectra at T =300 K with excitation energies: 6:9 eV (solid line), 7:2 eV (dash–dot–dot), 7:5 eV (dashed) and 7:7 eV (dash–dot). In the inset, the spectra at the same excitation energies are reported for T = 10 K.
is not done because the spectra are distorted by the high absorption of the sample. In Fig. 1, emission spectra at T = 300 K are reported for excitation energies in the v-UV range. It is observed that the band maximum position is subjected to small shift together with small variation of the FWHM on increasing the excitation energy. Analogous results are found for the E band. Besides, both bands feature a reduction of amplitude on increasing the excitation energy. Measurement at 10 K are reported in the inset of Fig. 1. Small variation of the maximum position and the FWHM are evidenced also at this temperature. The PL amplitude has non-monotonic dependence on the excitation energy as is well evident for the E band. Fig. 2 shows the position of the band maximum as a function of the excitation energy in the v-UV range from 6.9 up to 7:7 eV. At T = 300 K the position varies from 3.15 up to 3:18 eV and the FWHM (data not shown) changes from 0.44 to 0:46 eV. At T =10 K, we &nd that the maximum position changes from 3.13 to 3:15 eV and the FWHM changes from 0.36 to 0:39 eV. In the inset of Fig. 2, the dependence of the maximum position of the band is reported as a function of the excitation energy in the UV range from 4.6 up to 5:5 eV. Both at T = 300 and 10 K the maximum position changes almost monotonically from 3.07 up to 3:20 eV. At T = 300 K the FWHM increases from 0.38 to 0:51 eV increasing the excitation energy. At T = 10 K, where the band is clearly detectable for excitation energies from 5.0 up to 5:5 eV, FWHM increases from 0.38 to 0:46 eV. The dependence of the E band on the excitation energy is reported in Fig. 3. At T = 300 K, the position of the maximum changes in the range from 4.31 to 4:33 eV and the
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
η (area αE/ area β)
0.65 3.15
3.10
4.6
4.8
5.0
3.20
5.2
5.4
Eexc (eV)
η (area αE/ area β)
3.25
0.70
3.20
Peak Energy (eV)
Peak energy (eV)
3.30
647
0.60 0.55 0.50 0.45 0.40
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
7.0
7.2
7.4
7.6
Eexc(eV)
0.35 0.30 0.25
3.15
0.20 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
Eexc(eV) Fig. 2. Energy position of the emission maximum of the (blue) band reported as a function of the excitation energy in the vacuum-UV range at T = 300 K (triangles) and at T = 10 K (squares). In the inset, the position of the emission maximum is reported as a function of the excitation energy in the UV range at the same temperatures.
Peak energy (eV)
4.45
4.40
Peak Energy (eV)
4.40 4.35 4.30 4.25 4.20
4.6
4.8
5.0
5.2
5.4
Eexc (eV)
4.35
Eexc(eV) Fig. 4. Ratio, , of the area of the E and emission bands as a function of the excitation energy in the vacuum-UV range at T = 300 K. In the inset, is reported for T = 10 K.
position of the maximum of the E band shows much larger dependence on the excitation energy in the UV range featuring a non-monotonic variation both at T = 300 and 10 K. At variance, the FWHM increases from 0.46 to 0:61 eV at T = 300 K and from 0.38 to 0:56 eV at T = 10 K on increasing the excitation energy. To complete the characterization of the emission of our sample, we report in Fig. 4 the area ratio, , of the E band and the band. As shown, &rst increases and then decreases on increasing the excitation energy in the v-UV range at T = 300 K. At variance, at T = 10 K, increases and then shows a plateau. Under excitation energy in the UV range we found that, on increasing the excitation energy, decreases from 0.75 to 0.3 at T = 300 K, and from 300 to 18 at T = 10 K. 4. Discussion
4.30 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7
Eexc(eV) Fig. 3. Energy position of the emission maximum of the E (UV) band reported as a function of the excitation energy in the vacuum-UV range at T = 300 K (triangles) and at T = 10 K (squares). In the inset, the position of the emission maximum is reported as a function of the excitation energy in the UV range at the same temperatures.
FWHM from 0.49 to 0:53 eV. At T = 10 K the energy of the maximum changes from 4.32 to 4:35 eV and the FWHM from 0.44 to 0:50 eV. As reported in the inset of Fig. 3, the
The reported data evidence that under v-UV excitation both the E and bands are detectable at 300 as well as at 10 K. Fine tuning in the range 6.9 –7:7 eV shows that the area ratio of the E and bands changes with the excitation energy. This result suggests that the ISC process that is responsible for the coupling of the excited states from which the E and emissions arise (Skuja, 1992), has a dependence on the excitation energy in the v-UV range as already observed for the UV range (Leone et al., 1999). In this respect, the ISC process re
648
S. Agnello et al. / Radiation Measurements 38 (2004) 645 – 648
composite e;ect on the internal conversion. In fact, at 300 K, &rst increases up to 0.44 and then decreases down to 0.20 whereas, at T = 10 K, always increases up to 1.61 on increasing the excitation energy. It is worth noticing that the ratio under v-UV excitation is typically lower than under UV excitation, especially at low temperature, where the variation of a factor 15 of has been explained with the quenching of the ISC process (Leone et al., 1999; Cannizzo et al., 2003). This result suggests that the higher excited states give a more eLcient intersystem crossing process. The detailed study of the spectral features of the E and bands as a function of the excitation energy in the v-UV range from 6.9 up to 7:7 eV has evidenced that small changes in the position of the maximum and the FWHM of emission bands occur as compared to the e;ects observed under UV excitation, both at low and high temperature. The excitation energy dependence should mainly arise from the inhomogeneity, since under UV excitation it is observed also at low temperature when the ISC is poorly e;ective (Leone et al., 1999; Cannizzo et al., 2003). As a consequence, it is guessed that luminescence excited from the higher energy levels in the v-UV range merges the spectral inhomogeneity of the system. In this respect, it is noted that the maximum position and the FWHM of the E and emission bands when excited in the v-UV range are in good agreement with the average of the values found with UV excitation. Our data can be interpreted assuming one type of optically active center a;ected by glass matrix inhomogeneity. In particular, for excitation energy in the UV range around 5 eV the main process a;ecting the luminescence is an intra center conversion sensible to inhomogeneity of environment. At variance, for excitation in the v-UV range above 6:9 eV a competitive non-intra center excitation occurs, maybe through the conduction band, that is less sensible to inhomogeneity, as suggested by Trukhin et al. (1999). 5. Conclusions We have studied the blue and UV luminescence in Ge-doped silica, arising from B-type activity, by &ne tuning the excitation energy in the vacuum-UV range at 10 and 300 K. We have found that the area ratio of the emission bands depends on the excitation energy whereas the bands spectral features are not a;ected. This result is di;erent from the &ndings for UV excitation energy and it suggests that, under v-UV excitation, inhomogeneity is somewhat merged by a process involving highest excited energy levels. Acknowledgements The authors acknowledge useful discussions with L. Skuja and A. Trukhin. They thank M. Kirm of the G. Zimmerer group and A. Paleari for the measurements time at the
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